In the boring of holes in a workpiece or tapping threads into a bore in a workpiece, in the use of set screws, bolts, reamers and dowels within corresponding bores, often the particular tool has broken off leaving nothing or very little projecting from the workpiece with the result that it is very difficult to remove the broken-off tool part from the bore of the workpiece. Various efforts have been made to remove such tools. Apparatus previously available has not been efficient in accomplishing the desired result.
In Machinery's Handbook. 1Oth Edition, dated 1941, printed in New York by The Industrial Press under the heading "Removing a Broken Tap," p. 1071, it is indicated as follows: "The best and quickest method of removing a broken tap is by using a tap extractor. A design that has been proved successful is equipped with projecting fingers which enter the flutes of the tap which is backed out of the hole by turning the extractor with a wrench. This extractor is adjustable so as to support the fingers close to the tap, even when the broken end is below the surface of the work."
One difficulty in the use of the conventional tap extractor has been the problem involved when the flutes are curved rather than straight. Further, tap extractors have not been effective in achieving a sufficiently tight grip upon the broken-off part such as would permit its rotation and withdrawal from the workpiece.
The Handbook further indicates another method which consists of adding, by electric arc welding, metal onto the shank of the broken tap up to or above the level of the work. Care must be exercised to prevent depositing metal onto the threads in the tapped hole. After the shank has been built up, the head of a bolt or nut is tacked to it, and then the tap may be backed out.
Other apparatus and methods for removing broken drills or the like from apertures in a workpiece are disclosed in one or more of the following prior art patents:
______________________________________ U.S. PAT. NO. Patentee Date Issued ______________________________________ 2,429,967 N. A. Sorensen October 28, 1947 2,684,526 J. D. Hoover July 27, 1954 2,750,821 A. R. Hilsinger June 19, 1956 3,161,090 S. B. McLellan December 15, 1964 3,279,047 J. M. Cumberland October 18, 1966 3,439,567 A. P. Denis April 22, 1969 3,529,497 D. G. Brooks September 22, 1970 4,138,909 S. Johnson February 13, 1979 ______________________________________
One of the difficulties in the use of some of the prior art disclosures has been in the use of a flux with a welding electrode. Often there is such accumulation of flux upon the interior of an area where a weld is to be applied that the build-up of the flux prevents a proper weld from forming between the broken tool within the bore and the tool for extracting the broken tool from the workpiece.
Previously, there have been problems using flux which was building up and grounding out the welding electrode. The problem existed of finding a suitable means for effecting proper dissipation of the flux or flux vapor as soon as it has performed its function, namely the elimination of porosity holes in the weld.
Welding within a confined area with no place for the flux vapor to go has been one of the difficulties. One reason for this was that welding rods presently available created too much flux and hindered welding internally. Thus, the dissipation or use of flux was one of the problems with prior art structures involving welding.
Presently only one effective way of removing a broken tap exists. The process is identified as EDM (electrical discharge machining). Only a small percentage of shops have an EDM machine. Most shops must send via truck the piece of metal that contains the broken tap. An EDM machine is very expensive and utilizes a carbon electrode that actually burns out the broken tap. The EDM machine is a large machine, six feet by eight feet average, and requires a full-time operator.